Shaped protein structures and their preparation



Patented Oct. 21, 1947 UNITED STATES PATENT OFFICE SHAPED PROTEIN STRUCTURES AND THEIR PREPARATION Gordon F. Biehn, Mill Creek Hundred, and- Edward T. Cline, Brandywine Hundred, DeL,

assignors to E. 1. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing.

Application January 11, 1944, Serial No. 517,854

6 Claims. (01. 18-54) 7 ditions which result in the production of fibers and the like having dry and wet tenacities of at.

least 1.2 and 0.6 grams per denier, respectively (i. e., dry and wet tenacities of at least 1.2 and 0.6 g./d., respectively), when tested on the Scott incline plane tester at 60% relative humidity (i. e., 60% R. H.) and 21 C. A still further object is to provide prolamine synthetic fibers and the like whose elastic recovery from stretch and modulus of elasticity are close to those of wool. Still another object is to provide a process for the manufacture of fibers and the like from prolamines having a relatively high degree of orientation, as shown by X-rays, and an elastic recovery which is higher than that of any other protein synthetic fibers hitherto made. A still further object is to provide methods for producing such fibers. Additional objects will become apparent from an examination of the following description and claims. 1

These and other objects'and advantages are accomplished by the herein described invention which broadly comprises extruding an aqueous alkaline solution of a prolamine into an aqueous strong mineral acid coagulating bath containing formaldehyde and a salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts, partially hardening the resultant formed prolamine filaments by contacting said filaments with an aqueous solution of a mixture of a protein tanning agent and a salt material selected from the group con-- sisting of water-soluble inorganic salts and admixtures of said salts, stretching the resultant partially hardened prolamine filaments at least 50% of their maximum draw ratio in a concentrated aqueous solution of a salt material selected from the group consisting of water-soluble neutral and acid-reacting salts of mineral acids and admixtures of said salts, said concentrated solution having a temperature in excess of 50 C., and then immersing the resultant stretched filaments 2 in an aqueous formaldehyde hardening solution, whereby hardening of said filaments is completed.-

A"; preferred embodiment of this invention comprises extruding an aqueous zein solution containing from 12% to 18% zein by weight, from 1.5% to 3% sodium hydroxide. based on the weight of the zein, and about 5% formaldehyde, based on the weight of the zein, into an aqueous coagulating bath having a temperature within the range of from 30 C. to 70 C. and containing from 0.5% to by weight of formaldehyde, from 0.5% to 5% by weight of sulfuric acid, at least 10% by weight of sodium sulfate, and an amount within the range of from 0.01% to 0.5% by weight of a cation-active surface-active agent, collecting the resultant formed zein filaments on a bobbin, partially hardening said filaments by immersing them in an aqueous solution containing at least 5% by weight of a mixture of formaldehyde, aluminum sulfate and sodium chloride,

said aqueous solution containing at least 1% by weight of formaldehyde, at least 1% by weight of aluminum sulfate and at least 1% by weight of sodium chloride, stretching the resultant partially hardened zein filaments an amount within the range of from 50% to of their ,maximum draw ratio in an aqueous sodium sulfate solution having a temperature within the range of from 70 C. to 110 C. and containing from 10% to 30% by weight of. sodium sulfate, and then immersing the resultant stretched filaments in an aqueous formaldehyde hardening solution having a pH of not more than about 7 and containing at least 10% by weight of salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts.

By the term strong mineral acid as employed herein and in the appended claims is meant a water-soluble inorganic acid having a dissociation constant greater than 1x10 By the expression water-soluble as used herein and in the appended claims is meant a compound which is soluble in water to the extent of at least 5 grams per cubic centimeters. of water at 20 C.

' By the term inorganic salt as used herein and in the appended claims is meant a salt of a metal and an inorganic acid.

The maximum draw ratio of a fiber or the like, which is being stretched between rotating rollers, is the ratio between the-peripheral speed of the faster roller and the peripheral speed of the slower roller, when the rollers are operating at such speeds that the fiber is being stretched to a point just below that at which it will break.

By the term neutral salt as employed herein and in the appended claims is meant a salt which, when dissolved in water, provides a solution having a pH of about 7; while by the expression acid-reacting salt dissolved in water, produces a solution having a pH of less than 7.

Tenacities and, elongations of multifilament yarns are given in terms of measurements made with a constant specimen-rate-of-load-type testing machine '(Scott incline plane tester model 1P2) using a loading rate of 4 grams per denier per minute (A. S. T. M. designation: D25B-42) and a distance between clamps of four inches. One end of the conditioned yarn specimen is fastened in the carriage clamp, from the left hand clamp. The other end of the yarn is then passed through th left hand clamp and over the roller on the tension device. Sufficient tension is applied to the yarn end to balance the tension device and the left hand clamp is fastened. The machine is started and allowed to trace a stress-strain curve for the yarn on the chart provided. When the yarn breaks, a mark is made on the chart showing the position of the pen at the time of break. From the location of the point the tenacity and elongation of the specimen may be read on the chart. Tenacities are based on original yarn dimensions. When tested similarly on this machine, a sample of recent, 200 denier, 35 filament, 3 twist, textile grade, oiled, viscose rayon is found to have tenacities of 1.9, 0.9 and 1.6 g./d. and elongations of 24, Y26 and 16% dry, wet andloop respectively at 60% R. H. and 21 ,C.

Orientation as shown by X-rays is not to be confused with that shown by birefringence measurements and other measurements involving the use of polarized light. A low degree of orientation may be detected by means of birefringence measurements but the orintation must be of a relatively high order before it can be detected by means of X-rays. Orientation is measured by X-rays by exposing the fiber specimen to filtered copper radiation during one hour with a distance of 5 cm. between the specimen and the plate. The X-ray negative is developed and analyzed photometrically. From the density of silver deposit at any given point in the negative it is possible to calculate the intensity of the X-rays impinging on that point during the exposure. The extent of orientation of the specimen is expressed in terms of the average ratios of the calculated X-ray intensities at the equator of the picture to that at 90 degrees from the equator for the inner and outer rings; These ratios are called orientation numbers. Thus, an orientation number of one denotes that the specimen shows no orientation,

. whereas ratios progressively greater than one denote progressively higher degrees of orientation. By the term cation-active surface-active agent as employed herein and in the appended claims is meant an organic compound or admixture of organic compounds which in aqueous systems dissociate to yield positively charged surface-active cations. Compounds which are particularly adapted for this purpose are quaternary ammonium, quaternary phosphonium and tertiary sulfonium compounds which contain a positively charged ion that contains an aliphatic hydrocarbon group having at least six carbon atoms. Examples of such compounds are disclosed, for instance, in U. S. Patent 2,123,740.

is meant a salt, which when located four inches In the process of this invention the preparation of the spinning solution is accomplished by agitating the prolamine with water for several minutes. Then the alkali in the form of a concentrated aqueous solution is added with thorough agitation. The solution becomes homogeneous in about 90 minutes at room temperature. Homogeneity is accomplished in a shorter time if the solution is continuously agitated. Before spinning it is customary to filter the solution and deaerate it under vacuum to avoid stoppages in the spinneret and discontinuity in the spinning.

In the process of this invention the usual gear pump, candle filter, and viscose type spinneret are satisfactory for use. The number of orifices in the spinneret has little effect, except on the rate of fiber production, so that spinnerets containing either one or a multiplicity of orifices may be used. From the spinneret the solution is extruded directly into a coagulating bath containing chemicals which facilitate the transformation of the solution from a liquid fiber to a solid fiber.

A suitable bath may be described as one having a density greater than 1.1 and containing inorganic salts such as sodium sulfate or sodium chloride, a strong mineral acid, formaldehyde, and a small amount of cation-active surface-active agent.

The filaments are collected from the coagu1ating bath on to a bobbin and given a preliminary hardening treatment by immersion in an aqueous formaldehyde and salt containing bath such as one containing 7.6% sodium chloride, 3.9% aluminum sulfate, and 3.3% formaldehyde. The filaments are then stretched to at least 50% of their maximum draw ratio in a concentrated aqueous solution of a water-soluble inorganic salt at a temperature above 50 C., and the stretched fibers are passed into a final hardening bath containing formaldehyde and water-solubl inorganic salts.

The following examples are given for illustrative purposes'and are not intended to place any restrictions or limitations on the hereindescribed invention. In said examples all percentage quantitles of the prolamine are based on its dry weight. Measurements and proportions of all chemicals are given in terms of anhydrous rather than hydrated weight.

. Example I 20-60 lbs. pressure through an assembly of filter' cloth and wire screen. The filtrate was centrifuged to aid in removing the air bubbles. Deaeration of the solution was completed by subjecting it to reduced pressure at'about 0.5 lb./sq. in. absolute for several hours. At the time of spinning the solution had a viscosity of 76 viscose seconds at 30 C. and a pH of 11.55.

The solution was spun in ordinary viscose spinning equipment. The solution was fed under 15- lbs./sq. in. air pressure to a gear pump which forced'the material through a candle filter and spinneret into the aqueous coagulating bath. The spinneret had 40 orifices, each of which had a diameter of about 0.004 The coagulating bath contained 20% sodium sulfate, 4% zinc sulfate, 4% glucose, 3% formaldehyde, 2% sulfuric acid, and 0.05% cetyl pyridinium bromide. The co- 'to rotate about vertical shafts.

bath was of the order of 200 inches.

agulating bath was maintrined at a temperature of 30 C., and in traversing the length of the bath the yarn covered a distance of about 24 inches; The yarn leaving the bath was" collected on a bobbin, and when sufiicient yarn had been wound on the bobbin, the bobbin was removed from the wind-up machine and immersed in an aqueous solution containing 7.6%.sodium chloride, 3.0% aluminum sulfate, and 3.3% formaldehyde for about 16 hours at room temperature.

After this preliminary hardening treatment the fibers were stretched. This was done by removing the yarn from the bobbin by a positively driven roller and leading the yarn from the positively driven roller under a pulley immersed in an aqueous 20% sodium sulfate solution maintained at 86-88" C. and thence to a wind-up bobbin. The wind-up bobbin was operated at such a speed that the yarn was stretched about 400% 'dilute ammonium hydroxide solution, and allowed to dry on the bobbin.

The forty-filament yarn thus prepared had a denier of about 100, tenacities of 2.17, 1.06, and 1.61 g./d. dry, wet, and loop, respectively; elastic recovery from 4% stretch of 98%, elastic recovery from 8% stretch of 89%, and elastic modulusof 33 g./d., all tests being carried out at 60% R. H. and 21 C. These properties are much higher than those of prior art commercial protein synthetic fibers or those of protein fibers produced by any previously known process. In fact, the best prior art casein synthetic fibers, available in the form of staple fibers only, have markedly lower elastic recoveries, and tenacities of less than 1.0 and less than 0.4 g./d. dry and wet, respectively at 60% R. H. and 21 C. In addition, these zein fibers had an orientation number, as measured by X-rays, of 1.67, the highest orientation number yet shown by a protein synthetic fiber. Samples of prior art commercial casein fibers examined with X-rays similarly have shown orientation numbers of from 1 to 1.05, denoting substantially no orientation.

Example II The .spinning solution described in-Example I was spun into the same coagulating bath except that in this case six rollers were used in the coagulating bath. Each of these rollers had a circumference of 8.1. cm. in the groove, and three of them were mounted at each end of the coagulating bath in such a manner that they were free The yarn was led around these rollers and wound up on a bobbin. "I'he length of yarn travel in the coagulating The use of these rollers in the coagulating bath accomplished some stretching, probably about 50-100% in extent. In this case, the coagulating bath was maintained at a temperature of 40 C.

, As in Example I, the yarn was given a prelim- 6 bath comprising 20% sodium chloride, 1.1% sodium acetate, and 1.9% formaldehyde. The fibers were thoroughly washed with water, neutralized with dilute ammonium hydroxide, and dried on the bobbin. Yarn thus prepared had a denier. of 102 and dry, wet, and loop tenacities of 2.06, 0.93, and.1.56 g./d., respectively. Such tenacities are higher than those reported for any other globular protein fiber,

It is to be understood that the hereinbefore disclosed specific embodiments of this invention may be subject to variation and modification without departing from the scope thereof. However, it is critical to the obtaining of the new products of this invention, and more particularly tothe production of prolamine fibers and the like possessing a dry tenacity of at least 1.2 g./d., and a wet tenacity of at least 0.6 g./d. that:

(1) The-filaments be spun from an aqueous alkaline prolamine solution containing from 10% to 20% of the prolamine and having a pH of at least 9; v

.(2) The coagulation be, effected by extruding the prolamine solution into an aqueous. bath containing at least 0.1% by weight of formalde- I hyde, from 0.5% to 10% by weight of acid selected from the group consisting of strong mineralacids andadmixtures ofsaid acids, and at least 10% by weight of a salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said'salts;

(3) The filaments be given a preliminary.

- organic salts and admixtures of said salts, said in an aqueous formaldehyde hardening bath inary hardening treatment before stretching.

Stretching was carried out as in Example I, in this case to the extent of 440%. Final hardening of the yarn was carried out in an aqueous solution having a temperature of 50 C.;

(5) The stretched fibers be finally hardened more than having a pH oi? less than 8 and containing at least 10% by weight of salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts.

If the above procedure is adhered to, but not "otherwise, the fibers obtained not only will have high strength but also. surprisingly enough, will possess a high degree of elastic recovery-and hav an orientation number, as measured by X-rays, of at least 1.25.

Examples of prolamines useful in the practice of this invention are gliadin of wheat, hordein of barley, and zein of corn. Any good commercial grade of the aforementioned prolamines is satisfactory. Zein is the preferred prolamine becaus of its ready susceptability to the process of this invention, and because it is the best known and most readily available prolamine.

The basic agents used in the preparation of the alkaline prolamine solutions of this invention are water-soluble alkaline reacting compounds such as inorganic bases, including caustic alkalies and basic salts of alkali metals and ammonia, and organic bases such as amines, quaternary ammonium hydroxides, and tertiary sulfonium hydroxides. The strong bases such as especially suitable. Of these, because of its ready availability, sodium hydroxide is preferred The concentration of prolamine in the spinning solution and the ratio of prolamine to alkali are preferably regulated in order to give solutions having suitable viscosities for spinning and yielding fibers having optimum properties. Generally the optimum viscosities range between 20 and 200 poises. Solutions having suitable viscosities may be prepared either by using relatively low prolamine and alkali concentrations or. by using relatively high concentrations of both prolamine and alkali. However, the prolamine contert of the spinning solution should be within the range of from to 20% by weight.

Spinning solutions may be prepared containing from 10% to 20% by weight of the prolamine availability and the superior products had therewith.

The coagulating bath should contain a high percentage of a water-soluble inorganic salt or admixture of water-soluble inorganic salts. The

minimum salt content necessary is of the order of about 10% by weight; while the upper limit is determined by the solubility of the inorganic and from 1.5 to 5% alkaline reacting compound based on the weight of the prolamine. The pre-- ferred limits vary with the prolamine being used since the various prolamines display different solubility characteristics. Thus, for sodium hydroxide solutions of zein the optimum range for the dry prolamine content of the spinning solution is from 12% to 18% and for the sodium hydroxide content, based on the prolamlne, it is from 1.5% to 3%. The amount of alkaline reacting compound used varies to some extent depending upon the conditions obtaining during the isolation of the prolamine. It is essential, however, that the prolamin solution should contain alkaline reacting material in amount sufficient to provide a pH within the range of from 9 to 13. The novel fibers and the like of this invention are not had unless the prolamine spinning solution, employed in the process of this invention, has a pH within said range; while protein fibers and the like having optimum properties are produced when the pH of said spinning solution is within the range of from 10 to 12.5.-

It is to be understood that in addition to prolamine and water-soluble alkaline reacting compound, the spinning solution may contain a small percentage of formaldehyde, for instance from 1% to 10% formaldehyde based on the weight of the prolamine. Prolamine fibers and the like having optimum properties are obtained when said solution contains about 5% formaldehyde based on the weight of the prolamine; and for this reason the use of spinning solutions containing about this amount of formaldehyde is preferred.

In order to avoid extensive alkaline degradation of the prolamine, it is preferred to spin as soon after the solution becomes homogeneous as possible. Spinning of the aforementioned aqueous alkaline prolamine solutions is effected by extruding the prolamine solution into an aqueous coagulating bath. C'oagulating baths containlng formaldehyde, from 0.5% to 10% by weight of a strong mineral acid or admixture of strong mineral acids, and a high percentage of a water-soluble inorganic salt or an admixture of such salts are used.

While appreciable effects are obtained when the coagulating bath contains as little as 0.1% formaldehyde by weight, best results are obtained only when the formaldehyde content is at least 0.5% and preferably within the range of 0.5% to 10% by weight.

Though appreciable effects are obtained when the coagulating bath contains as much as 10% of a strong mineral acid, it is preferred, on account of the superior products thereby obtained,

salts in the coagulating bath composition. While any water-soluble inorganicsalt is adapted for use in the coagulating bath, water-soluble metal sulfates provide superior coagulating baths and hence are preferred. Examples of said metal sulfates include aluminum, aluminum potwsium, aluminum sodium, magnesium, potassium, sodium and zinc sulfates. The salts which are most readily available and which provide most satisfactory coagulating baths are the sulfates of sodium, zinc and aluminum.

Coagulating bath temperatures of 20-80 C. may be used. However, it is preferred to use temperatures within the range of from 30 C. to 70 C. since optimum filament properties result when temperatures within this range are employed. If too low temperatures are used, fiber properties are below normal and the salts in the bath tend to crystallize. At higher coagulating bath temperatures the tendency of spinnerets to become fouled and plugged is increased. Although it has been found that this tendency may be minimized by incorporating less than 1% of a cation-active surface-active agent in the coagulating bath, it is still an important factor in determining the upper temperature limit.

As hereinbefore stated, improved results are had when the coagulating bath contains at cation-active surface-active agent in amount which is less than 1% of the weight of said bath. For optimum results the-cation-active surface-active agent content of said bath is within the range of from 0.01% to 0.5% by weight.

The preliminary hardening treatment is an important feature of the process of this invention. It is essential for the attainment of the objects of this invention that the bath used be of such composition that it brings about a partial hardening or tanning of the fibers within the time of the treatment. The bath must therefore contain as a necessary component a protein tanning agent, e. g., formaldehyde, aluminum salts, chromium salts, quinones, and the like, and, in addition, water-soluble inorganic salts to repress swelling of the fibers. Optimum fiber properties are produced by the use of a preliminary hardening bath which is an aqueous solution of a mixture of formaldehyde, a water-soluble neutral inorganic salt and a Water-soluble acid-reacting inorganic salt. Saidsolution should contain at least 5% by weight of said mixture; and should contain at least 1% by weight of formaldehyde, at least 1% by Weight of Water-soluble neutral inorganic salt, and at least 1% by weight of water-soluble acid reacting salt. In general, the treatment is effected at ordinary room temperatures. If desired, however, elevated temperatures can be used. The time of the treatment depends upon the temperature used; thus, at ordinary temperatures of the order of about 20 C. to 25 C. the treatment requires up to 20 hours whereas at elevated temperatures it can be attained in about 30 minutes.

The coagulated and partially hardened filaments are subjected to stretching in a hot concentrated aqueous solution of a water-soluble neutral or acid-reacting salt of a'mineral acid or admixture of said salts. While any water-soluble salt of a mineral acid which in water provides a solution having a pH of not more than about 7, e. g.. aluminum sulfate, zinc sulfate, ammonium sulfate, is adapted for use in the stretching bath, it is preferred, on account of the superior fibers and the like formed therewith. to employ a stretching bath comprising a sodium salt of a strong mineral acid or admixtureof said salts. Sodium chloride and sodium sulfate provide fibers and the like having most desirable properties and are therefore preferred salts. The use of alkaline-reacting salts, especially the strong alkaline salts, is to be avoided; but the stretching bath may contain water-soluble salts of organic acids, e. g., sodium acetate and potassium formate, if desired. Generally, the salt concentration should be above and the temperature above 50 C. Preferred salt concentrations lie within the range of from to 30% and temperatures within the range of from 70 C. to 110 C. Temperatures in excess of 50 C. are necessary in order to obtain appreciable stretching. In the stretching step, other things being equal, higher degrees of stretch generally result in higher dry and ,wet tenacities in the final product. However, excessively high degrees of stretch cause frequent yarn and filament breakage. The preferred amount of stretch for a ty ical yarn is within the range of from 50% to 90% of the maximum draw ratio. Said maximum draw ratio varies to a certain extentfrom fiber to fiber depending upon the specific kind of prolamine employed, and the conditions obtaining during the coagulating and preliminary hardening ste s. but it may readily be determined by experimentation.

After the filaments have been coagulated, partially hardened and stretched, they are given a final hardening treatment and then washed. Washing may be accomplished by immersing the filaments in'the form of a hank in circulating water. The final formaldehyde insolubilization of the filaments may be effected in any known manner by contacting said filaments with an aqueous solution of formaldehyde containing at least 10% by we ght of a water-soluble inorganic salt or admixture of said salts and having a pH of less than 8. For optimum results said aqueous solution should contain at least 1% formaldehyde by weight. One preferred procedure involves hardening the fibers in an aqueous bath containing at least 1% formaldehyde by weight, sufllcient water-soluble inorganic salt to saturate the solution and a sufilcient amount of sodium acetate to give a pH between 6 and 7. Another preferred procedure comprises hardening the fibers in an aqueous bath containing on a weight basis at least 2% of formaldehyde and at least 10% of an admixture of a water-soluble neutral inorganic salt and a water-soluble acid-reacting inorganic salt.

It is to be understood that the formaldehyde which is employed in the hardening baths of this invention may be added to said baths as such or it may-be added in the form of materials which 1o methylolurea, trimethylolnitromethane and th like.

After the final hardening treatmentthe fibers and the like may be washed either under tension or in a relaxed condition until substantially free of acid and salts, and then dried by methods well known to the art.

In order that the filaments have high resistance to hot dilute acid baths such as are encountered instance, instead of formaldehyde itself, there may be used paraformaldehyde, trioxymethylene, di-

If desired, the fibers can be set before the final hardening step by treatment at an elevated temperature in a medium which has little swelling action on the fibers.

The fibers produced in accordance with the process of this invention are superior to prior art protein fibers in dry and wet tenacities, orientation numbers, elastic recovery from stretch and modulus of elasticity. As a consequence said fibers are useful as textile materials. They may be used to prepare high grade fabrics such as are used for dresses and suits.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the appended claims.

Having described the present invention, the following is claimed as new and useful:

1. In a process for obtaining prolamine synthetic fibers and the like, the steps of extruding an aqueous alkaline solution of a prolamine into an aqueous coagulating bath containing formaldehyde, acid selected from the group consisting of strong mineral acids and admixtures of said acids, and salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts, contacting the resultant formed prolamine filaments for a sufficient time to effect partial hardening with an aqueous solution containing at least 5% by weight of a mixture of a protein tanning agent and a salt material selected from the group consisting of watersoluble inorganic salts and admixtures of said salts, stretching the resultant partially hardened filaments at least 50% of their maximum draw ratio in a concentratedaqueous solution of salt material selected from the group consisting of water-soluble neutral and acid-reacting salts-of mineral acids and admixtures of said salts, said solution having a temperature of more than'50 C., and then immersing the resultant stretched filaments in an aqueous formaldehyde hardening.

solution.

2. In a process for obtaining prolamine synthetic fibers and the like, the steps of admixing a prolamine and an alkali with water in such proportions as to provide a solution containing from 10% to 20% by weight of prolamine and having a pH of at least 9, extruding the resultant aqueous alkaline prolamine solution into an aqueous coagulating bath containing at least 0.1% by weight of formaldehyde, from 0.5% to 10% by weight of acid selected from the group consisting of strong mineral acids and admixtures of said acids, and at least 10% by weight of salt material selected a ll from the group consisting of water-soluble inorganic salts and admixtures of said salts, immersing the resultant formed prolamine filaments for a suflicient time to efiect partial hardening in an aqueous solution containing at least by weight of a mixture of a protein tanning agent and a salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts, stretchin the resultant partially hardened filaments at least 50% of their maximum draw ratio in an aqueous salt solution having a temperature above 50 C. and containing at least 5% by weight of salt-material selected from the'group consisting of water-soluble neutral and acid-reacting salts of mineral acids and admixtures of said salts, and then immersing .the resultant stretched filaments in an aqueous formaldehyde hardening solution having a pH of less than 8 and containing at least 10% by weight of salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts.

3. In a process for obtaining prolamine synthetic fibers and the like, the: steps of extruding an aqueous sodium hydroxide solution of a prolamine, said solution containing from 10% to by weight of prolamine, sodium hydroxide within the range of from 1.5% to 5% based on the weight of the prolamine, and a small amount'of formaldehyde, into an aqueous coagulating bath having a temperature within the range of from 20 C. to 80 C. and containing at least 0.5% by weight of formaldehyde, from 0.5% to 10% by weight of sulfuric acid, at least 10% by weight of a water soluble metal sulfate, and up to 1% by weight of a cation-active surface-active agent, immersing the resultant formed prolamine filaments for a sufficient time to effect partial hardening in an aqueous solution containing at least 5% by weight of a mixture of formaldehyde, a water-soluble neutral inorganic salt and a water-soluble acidreacting inorganic salt, stretching the resultant partially hardened filaments at least 50% of their maximum draw ratio in an aqueous salt solution having a temperature of above 50 C. and containing from 10% to 30% by'weight of a water soluble sodium salt of a strong mineral acid, and then immersing the resultant stretched filaments in an aqueous formaldehyde hardening solution having a pH of not more than '7 and containing at least 10% by weight of salt material selected from the group consisting of water-soluble inorganic salts and admixtures of said salts.

4. The process according to claim 2 wherein the prolamine is zein.

5. The process according to claim 3 wherein the prolamine is zein.

6. In a process for obtaining zein synthetic flbers and the like, the steps of extruding an aqueous zein solution containing from 12% to 18% by weight of zein, and about 5% formaldehyde and sodium hydroxide within the range of from 1.5% to 3% based on the weight of the zein, into'an aqueous coagulating bath having a temperature within the range of from C. to 70 C. and containing from 0.5% to 10% by weight of formaldehyde, from 0.5% to 5% by weight of sulfuric acid, at least 10% by weight of sodium sulfate, and from 0.01% to 0.5% by weight of a cation-active surface-active agent, immersing the resultant formed prolamine filaments for a sufflcient time to effect partial hardening in an aqueous solution containing at least 5% by weight of a mixture of formaldehyde, sodium chloride and aluminum sulfate, stretching the resultant partially hardened filaments in an amount within the range of from 50% to 90% of their maximum draw ratio in an aqueous sodium sulfate solution having a temperature within the range of from 70 C. to 110 C. and containing from 10% to 30% by weight of sodium sulfate, and then immersing the REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Jacokes et a1 Jan. 18, 1944 Sturken July 22, 1941 Lamesch Mar. 9, 1943 Meigs Aug. 20, 1940 Ostenberg Sep. 23, 1919 Number Buron et al June 23, 1936 Swallen Jan. 18, 1938 Swallen et a1 Oct. 19, 1943 OTHER REFERENCES Swallen, Zein, a New Industrial Protein, Ind. and Engineering Chemistry, March 1941, pp. 394-398.

Osborne et a1 Jan. 28, 1902. 

